The disclosure of Japanese Patent Application No. 2000-191083 filed on Jun. 21, 2000, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a high-pressure fuel supply system for fuel injection in an internal combustion engine and to a method of supplying fuel.
2. Description of Related Art
In order to inject fuel directly into cylinders of an internal combustion engine, it is necessary to supply high-pressure fuel to fuel injection valves. High-pressure fuel supply systems for this purpose are known.
In general, a high-pressure fuel supply system has a reservoir leading to fuel injection valves, a high-pressure pump for force-feeding high-pressure fuel to the reservoir, and a low-pressure pump that is connected to the high-pressure pump on its intake side to ensure that the high-pressure pump withdraws fuel from a fuel tank. In general, the low-pressure pump is of an electrically driven type and can force-feed fuel at a rated discharge pressure since the starting of an engine, whereas the high-pressure pump is of an engine driven type. Because the internal combustion engine is driven by a starter motor and is at a low speed when it is started, the high-pressure pump cannot force-feed fuel well when the engine is started.
Thus, various propositions have been made including a proposition to boost a pressure in the reservoir to a rated discharge pressure (e.g., 0.3 MPa) of the low-pressure pump and to start fuel injection. However, this pressure is much lower than a target high fuel pressure (e.g., 12 MPa) in the reservoir during normal operation, and it is difficult to realize good fuel injection.
In order to solve this problem, Japanese Patent Application Laid-Open No. 5-321787 employs a pressure-boosting pump having a large-diameter piston and a small-diameter piston that are connected to each other in the axial direction. When starting an engine, a discharge pressure of a low-pressure pump is applied to the large-diameter piston so that the large-diameter piston and the small-diameter piston are displaced in the axial direction. Thus, the pressure of fuel in a small-diameter cylinder is boosted by the small-diameter piston by an amount corresponding to a ratio between pressure-receiving areas of the large-diameter piston and the small-diameter piston. It has been proposed to force-feed this pressure-boosted fuel to a reservoir that is connected to the small-diameter cylinder so as to boost a pressure in the reservoir to a pressure higher than a rated discharge pressure of the low-pressure pump.
Meanwhile, since fresh fuel is continuously supplied to the reservoir from a fuel tank while the engine is in operation, the temperature of fuel in the reservoir is lower than the temperature of a reservoir housing. However, after the engine has been stopped, fresh fuel is no longer supplied and the temperature of fuel in the reservoir becomes substantially equal to the temperature of the reservoir housing. Thus, immediately after the engine has been stopped, fuel in the reservoir receives heat from the reservoir housing, is heated up, and expands thermally. The reservoir is generally provided with a safety valve to prevent the pressure of fuel in the reservoir from rising above a predetermined level. Thus, the safety valve is operated by thermal expansion of fuel immediately after the engine has been stopped, and the pressure of fuel in the reservoir is maintained at a predetermined value.
After that, the temperature of the reservoir housing and fuel gradually falls to an outside air temperature. However, since fuel has a greater thermal expansion coefficient than the reservoir that is generally made from a metal, fuel thermally contracts more greatly than the reservoir housing in proportion to a fall in temperature. The pressure of fuel (i.e., the pressure in the reservoir) eventually becomes negative, and fuel vapors are generated in the reservoir.
In the case where such fuel vapors are generated in the reservoir, even if a pressure booster as described above is operated when starting the engine, some or all of the fuel that is force-fed from the small-diameter cylinder to the reservoir is used to eliminate the fuel vapors. Therefore, the pressure in the reservoir cannot be boosted as desired.
The invention has been made as a solution to the problem described above. It is thus one object of the invention to provide a high-pressure fuel supply system having a high-pressure pump, a low-pressure pump that can discharge fuel substantially at a rated discharge pressure since the starting of an engine, and a pressure booster for boosting a pressure of fuel in a reservoir to a pressure higher than a discharge pressure of the low-pressure pump when starting the engine, wherein the pressure booster can reliably boost a pressure in the reservoir when starting the engine even if there is a difference in thermal expansion coefficient between the reservoir housing and fuel.
A high-pressure fuel supply system according to one aspect of the invention comprises a reservoir for supplying fuel injection valves with high-pressure fuel, a high-pressure pump for force-feeding high-pressure fuel to the reservoir, a low-pressure pump that can discharge fuel substantially at a rated discharge pressure since the starting of an engine, a pressure booster that boosts a pressure of fuel in the reservoir when starting the engine, and a fuel passage that allows fuel to flow only from the fuel tank to the reservoir so as to prevent fuel vapors from being generated in the reservoir while the engine is out of operation.
Even if there is a difference in thermal expansion coefficient between a reservoir housing and fuel, the fuel passage prevents fuel vapors from being generated due to a negative pressure in the reservoir while the engine is out of operation. Thus, the pressure booster can reliably boost a pressure in the reservoir when starting the engine.
A high-pressure fuel supply system according to another aspect of the invention comprises a reservoir for supplying fuel injection valves with high-pressure fuel, a high-pressure pump for force-feeding high-pressure fuel to the reservoir, a low-pressure pump that can discharge fuel substantially at a rated discharge pressure since the starting of an engine, a pressure booster that boosts a pressure of fuel in the reservoir when starting the engine, and a delay device that delays operation of the pressure booster at least until fuel vapors in the reservoir are eliminated.
Other aspects of the invention involve methods of supplying high-pressure fuel.